Linux, all users: /etc/BC4Key.txt or /usr/lib64/beyondcompare/BC4Key.txt

After you've renamed the license key, run Beyond Compare, then select Help | About Beyond Compare from the menu on Windows/Linux
or Beyond Compare | About Beyond Compare on macOS.
Check Evaluate Pro Edition features.
Restart Beyond Compare.
When you are finished evaluating the Pro Edition, rename BC4Key.back to BC4Key.txt to restore your license.